`C = 1/21 + 1/77 + 1/165 + 1/285 + 1/437 + 1/621`
`C = 1/(3.7) + 1/(7.11) + 1/(11.15) + 1/(15.19) + 1/(19.23) + 1/(23.27)`
`4C = 4/(3.7) + 4/(7.11) + 4/(11.15) + 4/(15.19) + 4/(19.23) +4/(23.27)`
`4C = 1/3 - 1/7 + 1/7 - 1/11 + 1/11 - 1/15 + .... + 1/19 - 1/23 + 1/23 - 1/27`
`4C = 1/3 - 1/27`
`4C = 8/27`
`C = 8/27 : 4`
`C = 2/27`
Vì `2/27 < 4/27 => C < 4/27 ( đpcm)`